For hundreds of years, efforts have been made to harness the power of flowing fluids, and especially the natural flow of streams of wind and water over the surface of the earth. In this regard, various designs of windmills and waterwheels have long been known. In the industrial revolution and thereafter, more sophisticated turbines were developed to improve the efficiency of the extraction of power from the flowing fluid. Turbines were also developed to extract power from steam and hot gases as the flowing fluid. Such turbine apparatuses all include a rotor or runner mounted on a rotatable shaft, and may include a housing around the rotor and/or ducts delivering the fluid to and away from the rotor. The fluid flow is directed into and through the rotor, sometimes using stationary guide vanes or movable control gates, and energy is extracted from the flowing fluid by the rotor so as to impart rotation to the rotor shaft. Thereby, the rotating shaft transmits mechanical power that has been extracted from the flowing fluid. The mechanical power of the rotating shaft can then be used to carry out mechanical work (such as driving a mill, a pump, or the like) or it can drive an electrical generator or alternator to convert the mechanical power to electrical power. The conventionally known turbine and rotor designs can be categorized based on the orientation of the rotor axis relative to the flow of fluid. Cross-flow turbines have the fluid flowing through the rotor generally transversely to the rotor axis, which may be oriented horizontally, for example as represented by an undershot or overshot waterwheel, or vertically, with foil-shaped blades extending parallel to the axis in the general configuration of an egg beater. Axial flow turbines have the fluid flowing through the rotor substantially parallel to the rotor axis, which may be oriented vertically or horizontally. Such turbines may have a rotor in the configuration of a propeller with foil-shaped blades, or a helical screw, or a runner with shorter helical blades, for example represented by the Kaplan turbine. A further category of turbine has the fluid flowing radially outwardly through the rotor, for example as represented by the Fourneyron turbine. A further development of the radial flow turbines is represented by the Francis turbine in which the fluid flows radially inwardly into the rotor, and the rotor then deflects the fluid flow into an axial direction in which the fluid is exhausted or ejected from the rotor.
The known turbine configurations suffer from various disadvantages. The simpler devices like old-fashioned windmills and waterwheels have a very low efficiency and require large rotor dimensions to extract any usable amount of power from the flowing wind or water. The more advanced turbine designs with a rotor housing and ducts typically need to use movable flow control members that are actively controlled and thereby increase the complexity of the apparatus and are subject to malfunction and breakdown. For example, the Kaplan axial flow turbine uses adjustable pitch rotor blades, and the radial-axial flow Francis turbine uses controllable feed gates or guide vanes on the inlet side of the rotor. Furthermore, the more advanced turbine rotor designs typically require a relatively high pressure head of the flowing fluid. Therefore, for example Kaplan and Francis turbines are typically fed water with a relatively high pressure head from a barrage or impoundment reservoir behind a dam, as is known for large scale hydro power generation stations. The high water column created by such a water impoundment dam produces a higher static pressure of the water at the inlet side of the turbine, while the outlet side of the turbine discharges or exhausts the water substantially to normal ambient atmospheric pressure. There is thus a relatively high static pressure drop across the turbine system, whereby the inlet static pressure is converted partially to an increased velocity pressure and associated kinetic energy, which in turn is partially harvested as the water flows through the turbine rotor. Because they rely on the static pressure drop across the turbine system, such turbine and rotor designs, however, are not suitable for very low head applications, especially instream deployment of a turbine for extracting power from a fluid flow without barrage or impoundment of the fluid. In such instream applications, there can be no static pressure drop across the turbine system, because the fluid is taken from and returned to the ambient flowing stream at the ambient static pressure thereof.
It is becoming increasingly important to harness energy from a flowing fluid such as water or wind with a turbine apparatus that is deployed in the stream of the flowing fluid, without impounding the fluid behind a dam or the like. Such instream applications include harnessing wind power, ocean tidal power, or the power of a flowing river or stream, without damming the wind, the tidal flow or the stream flow. The damming of a stream or river destroys the river ecosystem upstream of the dam while potentially flooding large areas of previously existing upland, and can disrupt the natural habitat and spawning of fish and other wildlife in the river. Similarly, tidal barrage of the natural ebb and flow of the changing tides behind a dam in an estuary or sound, disrupts the natural marine ecosystem. Most of the commercially viable tidal power installations to this date involve such an estuary dam or tidal barrage. The tide flow is blocked, contained in one or more basins, and then released through turbines on the changing tide, thereby generating electrical power as the water flows out of the tidal barrage or basin. The side effects of such a tidal barrage have been environmentally undesirable or unacceptable, however.
Various turbine installations have also been developed for instream power generation, without the use of a dam. Instead, a turbine apparatus is installed underwater in the flowing stream, for example being fixably mounted on the seabed, or tethered to a pile in the seabed, or moored as a floating structure, or using hydrofoil-induced downforces to hold the device in the flowing stream of water. A portion of the kinetic energy of the water stream flowing through the blades of the turbine is then extracted or harvested by the rotating blades. Such instream turbines for harvesting tidal power include axial-flow horizontal or vertical axis turbines with a helical screw rotor (U.S. Pat. Nos. 7,147,428, 4,258,271) or a propeller-type or fan-type rotor (U.S. Pat. Nos. 7,471,009, 5,281,856, 4,258,271 4,163,904, US 2007/0241566), cross-flow vertical axis turbines with vertically extending hydrofoil blades or paddles (U.S. Pat. No. 6,499,939) or in which the water flows radially and tangentially into the rotor and axially out of the rotor (U.S. Pat. No. 4,686,376), cross-flow horizontal axis turbines in which the water flows radially and tangentially into the rotor and axially out of the rotor (U.S. Pat. No. 4,258,271), oscillating hydrofoils, and other designs. In such known turbine apparatuses, the rotor may be freely exposed to the ocean tidal current without any housing around the rotor (U.S. Pat. Nos. 7,199,484, 6,104,097, 4,026,587 2,501,696), or housed in a Venturi effect duct including a converging intake funnel leading to a constricted throat in which the rotor is arranged, and/or a diverging diffuser or exhaust outlet at the downstream end of the duct (U.S. Pat. Nos. 7,471,009, 7,147,428, 6,472,768, 4,258,271, 4,163,904, 3,986,787, US 2007/0241566). The converging and then diverging duct accelerates the water flow encaptured in the duct from its initial ambient flow speed to a higher speed at which it is directed through the rotor, and then decelerates the encaptured water to a discharge speed at or especially below the ambient flow speed. The reduction in the flow speed of the water represents the loss of kinetic energy, which was taken up or extracted by the rotor or lost in turbulence. The static pressure of the discharged water is equal to the ambient static pressure of the surrounding ambient stream.
Such instream power generating turbines suffer various disadvantages. For example, such instream turbines typically cannot extract power from all of the water that flows through them, because some of the water slips through the spaces between the turbine blades without doing any useful work. Furthermore, the most common instream turbines typically spin at a low rotational speed and therefore require expensive and complicated gearing arrangements to generate electrical power. The turbine blades must also be rather large, i.e. long, with a large swept area, in order to extract a useful amount of power from the low-speed low-head tidal water flow. The larger the turbine, the more difficult it is to develop a high rotational speed, because the tip speeds of the blades become extremely high even at a low rotational speed. For example, a turbine with 25 foot long blades spinning at only 50 rpm has a blade tip speed of 89 miles per hour. Such a turbine deployed instream in a tidal waterway represents a significant hazard to underwater life, including human swimmers or divers, as well as potential hazard or disruption to other commercial activities because shipping and fishing vessels must be kept far away from the turbine installation.
Various efforts have been made in the prior art to improve the instream turbine systems so as to avoid or ameliorate some of the above disadvantages. For example, as mentioned above, it is known to encapture the flowing fluid (e.g. water) in a duct or channel that includes a converging intake duct leading to the turbine and a diverging exhaust duct leading away from the turbine, so as to concentrate and accelerate the water flow through the turbine. Such ducts or channel housings make use of a Venturi effect to increase the speed and kinetic energy while reducing the static pressure of the flowing fluid, thereby increasing the efficiency of the turbine which harnesses some of the kinetic energy. As a result, such a ducted turbine can have a rotor of reduced size. Also, it is known that the duct outlet can have a larger cross-sectional area than the duct inlet, to correspond to the reduced flow velocity of the fluid at the outlet relative to the inlet (due to the extraction of kinetic energy from the fluid), further in consideration of the Betz limit.
Another known instream application for extracting power from a relative flow of water is to produce electrical power for use onboard a boat. Various turbine-driven generator devices are known, to be towed behind or beneath a boat as the boat sails or motors through the water (U.S. Pat. Nos. 6,508,191, 4,102,291). The relative motion between the boat (and thus the towed turbine) and the water causes the water to flow through the turbine, whereby some of the energy is extracted from the flowing water and powers the generator, so as to produce electricity for use onboard the boat. Such an application requires a much smaller size of turbine than the instream tidal power installations mentioned above, but the goal is nonetheless to achieve high efficiency in a smaller turbine apparatus arranged instream in a water flow. As mentioned above, in the prior art devices, it has been difficult to achieve high efficiency and high effectiveness with a small size turbine deployed instream in a fluid flow.
A major problem that has undermined the efficiency and effectiveness of all of the prior art instream devices, is that apparently no coherent and consistent design strategy has been applied to the entire apparatus from the intake inlet to the exhaust outlet, and especially to the design of the intake channel, the rotor channels and the exhaust channel to smoothly accelerate, redirect, and decelerate the water flow throughout its encapture, without causing power losses by turbulence and cavitation. Particularly, prior art turbines are subject to turbulence, cavitation and inefficient flow of the fluid through the channels and especially the rotor. Furthermore, prior art instream turbine arrangements have often required a large physical size of the rotor, which leads to a high cost because of the high demands on the material and the physical construction of the rotor. As also discussed above, such a large size generally leads to a slow rotational speed, which is not ideal for generating electricity, the most common goal of such instream power generation devices. Still further, the conventional turbine arrangements often use movable parts such as control vanes or gates, variable pitch rotor blades, pivotable paddles, pivotable hydrofoils, and the like. Such moving parts increase the complexity, the cost, and the need for maintenance and repair of the turbine installation.